setup: only allow Python 3 from a source checkout (issue5804) People are running `pip install Mercurial` with Python 3 and that is working because not everything performs a Python version compatibility check. Modern versions of pip do recognize the "python_requires" keyword (https://packaging.python.org/tutorials/distributing-packages/#python-requires) which we set if using setuptools. But this isn't set nor recognized everywhere. To prevent people from accidentally installing Mercurial with Python 3 until Python 3 is officially supported, have setup.py fail when run with Python 3. But don't fail if we're running from a source checkout, as we don't want to anger Mercurial developers hacking on Python 3 nor Mercurial's test automation running from source checkouts. People running setup.py from source checkouts could still fall through a Python 3 crack. But at least the `pip install Mercurial` attempt will get nipped in the bud.